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There are some files that should never be backed up. The main reason is that some files
do not contain anything that would matter if they were lost. Another reason is that some
files are representations of programs or data that are better backed up in a different
form. While virtually all regular files should be backed up, in most cases the following
kinds of files should be excluded from routine backups:

Swap Files: A swap file is a large file that is used by the operating system for
virtual memory. When the system needs more memory than actually exists in the PC, it
creates a virtual memory space and applications "share" the real memory by
swapping pieces of memory to the hard disk. The swap file holds these pieces; the process is described in much more detail here.
This file can be quite large, sometimes exceeding 50 MB in size depending on the system.
Since it does not contain any real data, but rather is a placeholder for information in
memory while the PC is running, there is no point in backing it up.

Compressed Volume Files: If you use volume-based disk compression, the compressed
volume that you see mounted as a drive letter is stored on the host disk as a single file
called a compressed volume file or CVF.
All the files on the compressed disk are in this file. If you use compressed volumes, the
files on them should be backed up individually from the compressed volume. The big CVF
file on the host disk should not be backed up, since it contains the same information but
in a way that is much more difficult to restore.

Most newer software will in fact automatically deselect the items above, unless you
override and tell it you want them included anyway. Many types of backup software will
also let you select classes of files, by file type, that you want to exclude for whatever
reason.